Various devices have ben proposed which provide for liquid dispensing. These generally fall into three groups, volume dispensing, timed open loop dispensing or gravimetric dispensing. Among the liquid dispensing devices are automated berets such as the device disclosed in U.S. Pat. No. 3,989,169. Other devices such as syringe drive mechanisms as disclosed in U.S. Pat. No. 5,055,263 have also been used to provide for delivery of a particular liquid. Certain of these systems provides a means of mixing two different liquids in a manner which results in the dilution of one solution with the other. For example, U.S. Pat. No. 4,734,378 is directed to the use of two syringe barrels and mixing. The syringes are independently activated and dispense both liquids at a rate which produces the desired dilution.
Both of these types of systems are prone to errors introduced by the presence of air bubbles, dissolved gasses and leaks.
Liquid dispensing based upon weight has been described in U.S. Pat. No. 4,350,186. Here, liquid is delivered to a container whose weight is measured by an electronic balance which is monitored by a computer. The pump is operated at a fixed delivery rate until the desired amount of liquid is delivered to the vessel. The pump is stopped or a valve is closed when the weight of liquid in the vessel equals the amount desired. Although gravimetric systems of this type overcome the problems commonly associated with volumetric delivery, repeated calibration is usually necessary to achieve the required accuracy and delivery is often slow, hampered by the need to constantly stop and check how much liquid is still needed. Further, systems utilizing this method risk over-shooting the target amount, if there is a significant delay from the time the weight is detected until the flow is stopped.
In addition, the precision and accuracy offered by both the volumetric and previously existing gravimetric procedures often are not suitable for use in a laboratory or industrial setting where more exacting specifications are required. There is, therefore, a long felt need for a system capable of accurately and precisely preparing aqueous and organic solvent based solutions of various salts, acids and bases.
This novel solution preparation system which is herein presented has demonstrated the ability to dispense liquids rapidly, precisely and accurately using closed loop feedback control. The problems associated with other gravimetric liquid dispensing systems have been avoided by incorporating classical closed loop feed back control techniques into the mechanical design.